Method and apparatus for reducing thermal resistance in a vertical heat sink assembly

ABSTRACT

A method and apparatus for an electronic package includes a substrate; a heat source component operably coupled to the substrate, and in direct contact with and electrically connected to a top surface of the substrate; a heat sink assembly in thermal communication with the substrate. The heat sink assembly includes a plurality of distinct vapor chambers, each containing a heat transfer fluid configured to evaporate on a wall in thermal contact with a back surface of the heat source component and condense on an opposing wall defining an exterior wall defining the vapor chambers. Each of the plurality of distinct vapor chambers are serially aligned having facing sidewalls defining each relative to contiguous vapor chambers and at least one of the plurality of distinct vapor chambers includes a lower sidewall defining one distinct vapor chamber substantially aligned with a bottom defining the heat source component such that a bottom portion defining the one distinct vapor chamber is substantially aligned with a bottom portion of the heat source component.

BACKGROUND OF THE INVENTION

The present invention relates to dissipating heat generated byintegrated circuit (IC) modules, and a method of constructing suchdevices. In particular, the present disclosure relates to a method andapparatus for eliminating a dry out condition of a heat transfer orcooling fluid in a vertical heat sink assembly configured to dissipateheat generated by integrated circuit modules.

As is known, operating electronic devices produce heat. This heat shouldbe removed from the devices in order to maintain device junctiontemperatures within desirable limits: failure to remove the heat thusproduced results in increased device temperatures, potentially leadingto thermal runaway conditions. Several trends in the electronicsindustry have combined to increase the importance of thermal management,including heat removal for electronic devices, including technologieswhere thermal management has traditionally been less of a concern, suchas CMOS. In particular, the need for faster and more densely packedcircuits has had a direct impact on the importance of thermalmanagement. First, power dissipation, and therefore heat production,increases as the device operating frequencies increase. Second,increased operating frequencies may be possible at lower device junctiontemperatures. Finally, as more and more devices are packed onto a singlechip, power density (Watts/cm²) increases, resulting in the need toremove more power from a given size chip or module. These trends havecombined to create applications where it is no longer desirable toremove the heat from modern devices solely by traditional air coolingmethods, such as by using traditional air cooled heat sinks.

For example, with the advent of multichip modules (MCMs), containingmultiple integrated circuit (IC) chips each having many thousands ofcircuit elements, it has become possible to pack great numbers ofelectronic components together within a very small volume. As is wellknown, ICs generate significant amounts of heat during the course oftheir normal operation. Since most semiconductor or other solid statedevices are sensitive to excessive temperatures, a solution to theproblem of the generation of heat by IC chips in close proximity to oneanother in MCMs is of continuing concern to the industry.

A conventional approach to cooling components in electronic systems inwhich devices contained in MCMs are placed on printed circuit/wireboards or cards is to direct a stream of cooling air across the moduleswith the addition of heat sinks attached to the module to enhance theeffectiveness of the airflow.

Limitation in the cooling capacity of the simple airflow/heat sinkapproach to cooling has led to the use of another technique, which is amore advanced approach to cooling of card-mounted MCMs. This techniqueutilizes heat pipe technology. Heat pipes per se are of course, wellknown and heat pipes in the form of vapor chambers are becoming common.In the related art, there are also teachings of heat pipes/vaporchambers for dissipating the heat generated by electronic componentsmounted on cards.

One approach includes using a cooling fluid or heat transfer fluid in avapor chamber heat sink. A vapor chamber base enables heat sinks toperform better as the thermal resistance to spreading the heat in thebase is reduced. The heat is removed from one side of the base inthermal communication with a heat source by evaporation of the heattransfer fluid and travels rapidly in a gaseous state until it condenseson a fin side of the base. In this manner, the heat is transferred fromthe base to the fins for subsequent conduction to convectively cooledfins extending from the base.

However, vapor chamber technology has several limitations when appliedto MCMs. One limitation is that the above described heat transfermechanism can fail if inadequate heat transfer fluid or cooling fluid ispresent on the evaporator surface of the base near the heat source. Thisis often the case when the vapor chamber is positioned vertically suchthat gravity causes the returning or condensed cooling fluid toaccumulate at a lower area of the vertically oriented vapor chamber. Forapplications where the heat source is centrally located with respect tothe vertically oriented heat sink, dry out conditions are often creatednear the heat source.

For the foregoing reasons, therefore, for an efficiently cooledelectronic module or MCM that employs vapor chamber cooling. Inparticular, there is a need in the art for a method and apparatus ofproviding a vertically oriented vapor chamber and corresponding heatsource to be cooled with a fluid coolant, while simultaneouslyeliminating dry out conditions near the heat source.

SUMMARY OF THE INVENTION

One embodiment is an electronic package includes a substrate; a heatsource component operably coupled to the substrate, and in directcontact with and electrically connected to a top surface of thesubstrate; a heat sink assembly in thermal communication with thesubstrate. The heat sink assembly includes a plurality of distinct vaporchambers, each containing a heat transfer fluid configured to evaporateon a wall in thermal contact with a back surface of the heat sourcecomponent and condense on an opposing wall defining an exterior walldefining the vapor chambers. Each of the plurality of distinct vaporchambers are serially aligned having facing sidewalls defining eachrelative to contiguous vapor chambers and at least one of the pluralityof distinct vapor chambers includes a lower sidewall defining onedistinct vapor chamber substantially aligned with a bottom defining theheat source component such that a bottom portion defining the onedistinct vapor chamber is substantially aligned with a bottom portion ofthe heat source component.

Another embodiment is a method for lowering a thermal resistance of avertically oriented heat sink assembly to dissipate heat from a heatsource component. The method includes configuring a heat sink assemblywith a plurality of distinct vapor chambers, each of the distinct vaporchambers containing a heat transfer fluid configured to evaporate on awall in thermal contact with a back surface of the heat source componentand condense on an opposing wall defining an exterior wall of the heatsink assembly; and configuring each of the plurality of distinct vaporchambers to be serially aligned having facing sidewalls defining eachrelative to contiguous vapor chambers and at least one of the pluralityof distinct vapor chambers includes a lower sidewall defining onedistinct vapor chamber substantially aligned with a bottom defining theheat source component such that a bottom portion defining the onedistinct vapor chamber is substantially aligned with a bottom portion ofthe heat source component.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring to the exemplary drawings wherein like elements are numberedalike in the accompanying Figures:

FIG. 1 depicts a perspective view of a partially populated centralelectronics complex (CEC) illustrating an exposed vertically orientedMCM and a vertically oriented heat sink assembly disposed over anotherMCM;

FIG. 2 depicts a partial exploded cross section view of the verticallyoriented heat sink assembly disposed over a MCM of FIG. 1;

FIG. 3 depicts a partial exploded cross section view of an exemplaryembodiment of a vertically oriented heat sink assembly having vaporchambers integrated into a base of a heat sink disposed over a MCM ofFIG. 1;

FIG. 4 depicts a schematic side view of a prior art vertically orientedheat sink assembly having a single vapor chamber in thermalcommunication with a centrally located heat source;

FIG. 5 depicts a schematic side view of a vertically oriented heat sinkassembly having two distinct vapor chambers in thermal communicationwith the heat source of FIG. 4 in accordance with an exemplaryembodiment of the present disclosure; and

FIG. 6 depicts a schematic side view of a vertically oriented heat sinkassembly having two distinct vapor chambers in thermal communicationwith the heat source of FIG. 5 in accordance with an alternativeexemplary embodiment of the present disclosure.

DETAILED DESCRIPTION OF THE INVENTION

The present disclosure will now be described in more detail by way ofexample with reference to the embodiments shown in the accompanyingfigures. It should be kept in mind that the following describedembodiments are only presented by way of example and should not beconstrued as limiting the inventive concept to any particular physicalconfiguration.

Further, if used and unless otherwise stated, the terms “upper”,“lower”, “front”, “back”, “over”, “under”, and similar such terms arenot to be construed as limiting the invention to a particularorientation. Instead, these terms are used only on a relative basis.

For the purposes of the present disclosure, the terms printed circuitboard (PCB) and printed wire board (PWB) are equivalent terms. The terms“in contact” and “contacting” indicate mechanical and thermal contact

FIG. 1 illustrates a so-called central electronics complex 10 (CEC) of acomputer system. The CEC 10 is comprised of an enclosure (such as a cage12), a backplane or midplane 14 as illustrated, and a circuit board ordaughter card, such as a blade or node 16 having two processormulti-chip modules (MCM) 17, and a corresponding vapor chamber heat sinkassembly 20 disposed over each MCM 17 (only one shown of each for sakeof clarity), 256 GB memory on 16 cards (not shown), an input/output(I/O) card 18, and a control multiplexer card (not shown), for example,attachable to the backplane 14. Air inlets 22 are shown at a back of CEC10 and air flow across fins of heat sink assembly 20 and out airexhausts 24 is generally shown with arrows 26.

FIG. 2 is a partial cross-sectional view of an exemplary embodiment of amultichip module (MCM) mounted on a PCB having a lid in accordance withthe present disclosure. It will be recognized by one skilled in thepertinent art that a dual chip module (DCM) is also contemplated for usein the present disclosure. In FIG. 2, MCM 100 includes a substrate 102having a multiplicity of components 105 mounted thereto, each componenthaving a front surface 110 and a back surface 115. MCM 100 is mounted toa PCB 120 by a multiplicity of solder balls 125. Substrate 102 may be asingle or multi-level substrate and may be ceramic, fiberglass orpolymer based. MCM 100 also includes a lid 130. Lid 130 is mounted tosubstrate 102 by lid support 132 connecting the periphery of lid 130 tothe periphery of substrate 102. Lid support 132 may be fabricated fromthe same material as lid 130 and may be integral with the lid.Alternatively, lid support 132 may be fabricated from a materialdifferent from that of lid 130. Lid support 132 may provide a hermeticseal between lid 130 and substrate 102.

Lid 130 includes a lower wall 135 having an outer surface 140, an upperwall 145 having an outer surface 150 and sidewalls 155 defining a vaporchamber 160. It will be noted that opposing sidewalls 155 defining vaporchamber 160 are shown closer together than with respect to FIG. 5 forsake of clarity in describing chamber 160. Moreover, FIG. 2 is shownwith a single vapor chamber 160, wherein two or more vapor chambers 160are included in an exemplary embodiment depicted in FIG. 5 in accordancewith the present disclosure.

Vapor chamber 160 contains a heat transfer fluid such as, inter alia,water, freon or glycol. Front sides 110 of components 105 areelectrically connected to a top surface 165 of substrate 102. Components105 may be flip chip, wire-bonded or soldered to substrate 102. Athermal transfer medium 170 is in contact with back surfaces 115 ofcomponents 105 and outer surface 140 of lower wall 135 of lid 130 toenable thermal contact, mechanical restraint and pressure support overthe contacting region. Thermal transfer medium 170 enables heatgenerated by the operation of components 105 to be efficientlytransferred to lid 130.

Because of the excellent heat transfer capability afforded to lid 130 byvapor chamber 160, the lid may be fabricated from many differentmaterials including but not limited to metals such as aluminum, copper,nickel, gold or Invar and other materials such as plastics, ceramics andcomposites. Because of the wide range of materials available, lid 130may fabricated from a material having a CTE matched to (between about25% to 700% of the coefficient of thermal expansion) substrate 102 orfrom the same material as the substrate. For example, if MCM 100 is aHyperBGA® International Business Machine Corp., Armonk, N.Y., in whichsubstrate 102 is a polytetraflouroethylene (PTFE) based material havinga CTE of about 10-12 ppm/° C., then lid 130 may be fabricated from analuminum-silicon carbide composite having a CTE of about 10 ppm/° C.

Thermal transfer medium 170 may include a thermal adhesive, thermalgrease, thermal-conductive pads, phase change or other materials knownin the art.

A heat sink 180 having a plurality of horizontal fins 182 (see alsoFIG. 1) is in thermal communication with outer surface 150 of lid 130.Heat sink 180 is shown remove from outer surface 150 for sake ofclarity. Heat sink 180 may be formed from aluminum, copper, beryllium,white metal or any other suitable material with high heat conductivity.Furthermore, it will be recognized by one skilled in the pertinent artthat heat sink 180 may be fabricated from a material having a CTEmatched to (between about 25% to 700%) the CTE of lid 130. Moreover, itwill be recognized that although heat sink 180 and lid 130 are shown asseparable parts, heat sink 180 may be integrated with lid 130 in asingle integral part.

In an exemplary embodiment referring to FIG. 3, vapor chamber 160 isintegrated in a base defining heat sink 180 while lid 130 is a solidsubstrate. It will be recognized by one skilled in the pertinent artthat having vapor chamber 160 integrated in a base of the heat sinkversus lid 130, enables the vapor chambers to spread the heat wellbeyond the confines of the cap of a module (e.g., lid 130). In anexemplary embodiment depicted in FIG. 3, a width of the heat sink vaporchambers are about twice the width of lid 130. In either case, it isnoted that the vapor chamber heat sink assembly 20 of FIG. 2 includes acombination of heat sink 180 and lid 130 or a base of heat sink 180having vapor chamber 160 integrated therewith, as in FIG. 3.

While MCM 100 has been illustrated in FIGS. 1 and 2 and described aboveas a ball grid array (BGA) module, MCM 100 may be pin grid array (PGA)module. Instead of solder balls 125 (see FIG. 2), Land Grid Array (LGA)connections between substrate 102 and PCB 120 are also contemplated.Since LGA connections are asperity contact connections, generally somedegree of pressure must be maintained on the connection to ensure goodelectrical conductivity. Therefore, flanges 184 defining ends of heatsink 180 may be used accept a mechanical fastener and engage substrate120 to provide the necessary pressure to ensure suitable electricalconductivity.

Referring now to FIG. 4, a vertically oriented heat sink 180 is inthermal communication with a prior art evaporator 200 having acorresponding vertically oriented single vapor chamber 260, which is inturn in thermal communication with a heat source 210. Heat source 210 iscentrally located with respect to a bottom surface 240 defining a lengthof evaporator 200. It will be recognized that heat source 210 may be MCM100 as described above. In many such evaporators, a heat transfer fluidgenerally indicated at 220 such as, inter alia, water, has a tendency toaccumulate at a bottom of vapor chamber 260 generally indicated at 224because of gravity acting on transfer fluid 220. The result isinadequate liquid to evaporate heat load proximate heat source 210 andevaporator 200 proximate the heat source 240 dries out as the heattherefrom is conducted further towards the water accumulation at 224,reducing thermal performance of the assembly. The thermal performance isreduced by the increase in thermal resistance due to an increase in pathlength for the heat to travel to heat sink 180 because of the dried outsection local to the centrally located heat source 210.

Referring now to FIG. 5, vertically oriented heat sink 180 isillustrated in thermal communication with an exemplary embodiment of anevaporator 300 defining at least two distinct vapor chambers 360, whichis in turn in thermal communication with heat source 210. Heat source210 is substantially centrally located with respect to a bottom surface340 defining a length of evaporator 300 as described with respect toheat source 210 in FIG. 3, however, heat source 210 may be alignedanywhere along a length defining bottom surface 340. It will berecognized that heat source 210 may be MCM 100 as described above. Sinceheat transfer fluid 220 has the tendency to accumulate at a bottom ofvertically oriented vapor chamber 260 in FIG. 4, vapor chamber 360 inFIG. 5 includes an upper vapor chamber 361 and a lower vapor chamber 362separated from upper chamber 361 via a horizontal barrier 370therebetween. Barrier 370 is configured to prevent condensed heattransfer fluid 220 from being pulled by gravity past heat source 210into vapor chamber 362. Barrier 370 extends from bottom surface 340 toupper surface 350, substantially normal to both.

In an exemplary embodiment as illustrated, barrier 370 is a horizontalsolid section separating vapor chamber 360 into two distinct chambers,361, 362. The larger upper vapor chamber 361 will have an ample supplyof heat transfer fluid available near heat source 210 in spite ofgravity acting thereon in vertical mount applications. Even though thelower vapor chamber 362 still works against gravity, lower vapor chamber362 still provides some added cooling. Vapor chambers 361 and 362together outperform a single vapor chamber in most vertical applicationsbecause adequate liquid is available and local to heat source 210 toevaporate heat load proximate heat source 210. Heat from heat source 210is less prone to dry out vapor chamber 361 since a bottom of vaporchamber 361 is substantially aligned with heat source 210. Inparticular, a bottom of heat source 210 substantially coincides with abottom of vapor chamber 361 where heat transfer fluid 220 would tend toaccumulate due to gravity. Thus, the heat transfer path is less likelyto increase because vapor chamber 361 insures that the local evaporatorarea is wet, thus lowering thermal resistance of heat transfer to heatsink 180.

The thermal performance is increased by lowering thermal resistance dueto a decreased path length for the heat to travel to heat sink 180because of eliminating a dried out section local to the centrallylocated heat source 210.

In an exemplary embodiment, upper vapor chamber 361 is configured havinga bottom portion thereof substantially aligned or alternatively, notextending much past heat source 210 insuring that the evaporator arealocal to the heat source 210 is wet with condensed heat transfer fluid220. The lower vapor chamber 362 works against gravity over a regionsimilar to that described with respect to the single vapor chamber 260in FIG. 3, but still provides some added cooling. The mean path for theheat to travel from heat source 210 to find condensed heat transferfluid is reduced by two or more separate vapor chambers definingevaporator 300, thus lowering the thermal resistance to heat sink 180.Although, upper vapor chamber 361 has been described as being larger,e.g., longer relative to vertical, than lower vapor chamber 362, vaporchambers may be configured having substantially the same length or lowerchamber 362 may be longer than upper vapor chamber 361.

For example, FIG. 6 depicts lower vapor chamber 362 configured longerthan upper chamber 361. In this case, it will be noted that heat source210 is then more efficiently disposed towards an upper portion defininga length of bottom surface 340.

Thus, an efficiently cooled IC, such as a MCM, that employs vaporchamber cooling with a plurality of separate vapor chambers in thermalcommunication with a vertically oriented heat sink assembly whileminimizing dry out conditions and reducing thermal resistance has beendescribed.

While the invention has been described with reference to exemplaryembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the invention isnot to be limited to the particular embodiment disclosed as the best oronly mode contemplated for carrying out this invention, but that theinvention will include all embodiments falling within the scope of theappended claims. Moreover, the use of the terms first, second, etc. donot denote any order or importance, but rather the terms first, second,etc. are used to distinguish one element from another. Furthermore, theuse of the terms a, an, etc. do not denote a limitation of quantity, butrather denote the presence of at least one of the referenced item.

1. An electronic package comprising: a substrate; a heat sourcecomponent operably coupled to said substrate, said component in directcontact with and electrically connected to a first surface of saidsubstrate; a heat sink assembly in thermal communication with anopposite second surface of said substrate, said heat sink assemblyincluding a plurality of discrete vapor chambers vertically aligned withrespect to each other, each of said plurality of discrete vapor chamberscontaining a heat transfer fluid in thermal contact with a back surfaceof said heat source component, said heat transfer fluid configured toevaporate from a bottom portion defining each of said plurality ofdiscrete vapor chambers and condense on an upper surface defining an topportion of each of said vapor chambers of said heat sink assembly; andwherein said each of said plurality of discrete vapor chambers areserially aligned having facing lower and upper surfaces defined by atleast one wall therebetween corresponding to contiguous vapor chambers,a lower surface defining one of said plurality of discrete vaporchambers being substantially aligned with a bottom defining said heatsource component such that the lower surface defining said one of saidplurality of discrete vapor chambers is substantially aligned with abottom portion of said heat source component.
 2. The electronic packageof claim 1, wherein said heat sink assembly includes a heat sink inthermal communication with an outer surface defining a lid attached tosaid substrate.
 3. The electronic package of claim 2, wherein said heatsink is integrally formed with said lid.
 4. The electronic package ofclaim 2, wherein said plurality of discrete vapor champers are separatedby a horizontal barrier extending from said lower surface to said uppersurface and defining contiguous vapor chambers.
 5. The electronicpackage of claim 1, wherein said plurality of vapor chambers includes anupper vapor chamber and a lower vapor chamber.
 6. The electronic packageof claim 5, wherein when said heat source component is substantiallycentrally located relative to a length defining said heat sink assembly,said upper vapor chamber is longer than said lower vapor chamber.
 7. Theelectronic package of claim 5, wherein when said heat source componentis located substantially above a central location relative to a lengthdefining said heat sink assembly, said lower vapor chamber is longerthan said upper vapor chamber.
 8. The electronic package of claim 1,wherein said package is selected from the group consisting of ball gridarray modules, pin grid array modules, land grid array modules andHyperBGA® modules.
 9. The electronic package of claim 2, wherein saidlid is formed from material selected from the group consisting ofaluminum, copper, Invar, gold, silver, nickel, aluminum-silicon carbide,plastics, ceramics and composites.
 10. The electronic package of claim1, wherein said substrate includes material selected from the groupconsisting of ceramics, fiberglass, polytetraflouroethylene, andpolymers.
 11. The electronic package of claim 1, wherein a solid thermaltransfer medium is in direct contact with a back surface of each heatsource component and an outer surface of a lower wall of said heat sinkassembly.
 12. The electronic package of claim 1, wherein said heatsource component is one of a DCM and a MCM.
 13. A method for lowering athermal resistance of a vertically oriented heat sink assembly todissipate heat from a heat source component, the method comprising:configuring a heat sink assembly with a plurality of discrete vaporchambers, each of said plurality of discrete vapor chambers containing aheat transfer fluid in thermal contact with a back surface of the heatsource-component, said heat transfer fluid configured to evaporate froma bottom portion defining, each of said plurality of discrete vaporchambers and condense on an upper surface a top portion of each of saidvapor chambers of said heat sink assembly; and configuring said each ofsaid plurality of discrete vapor chambers to be serially, verticallyaligned having facing lower and upper surfaces defined by at least onewall therebetween corresponding to contiguous vapor chambers, a lowersurface defining one of said plurality of discrete vapor chambers beingsubstantially aligned with a bottom defining the heat source componentsuch that the lower surface defining said one discrete vapor chamber issubstantially aligned with a bottom portion of the heat sourcecomponent.
 14. The method of claim 13, further comprising: disposing aheat sink in thermal communication with an outer surface defining lidattached to said substrate.
 15. The method of claim 14, furthercomprising: integrally forming said heat sink with said lid.
 16. Themethod of claim 14, further comprising: separating said plurality ofdiscrete vapor champers with a horizontal barrier extending from saidlower surface to said upper surface defining contiguous vapor chambers.17. The method of claim 13, wherein said plurality of vapor chambersincludes an upper vapor chamber and a lower vapor chamber.
 18. Themethod of claim 17, further comprising: locating the heat sourcecomponent in a central location relative to a length defining said heatsink assembly, wherein said upper vapor chamber is longer than saidlower vapor chamber.
 19. The method of claim 17, further comprising:locating the heat source component substantially above a centrallocation relative to a length defining said heat sink assembly, whereinsaid lower vapor chamber is longer than said upper vapor chamber. 20.The method of claim 13, further comprising: disposing a solid thermaltransfer medium in direct contact with a back surface of each heatsource component and an outer surface defining said heat sink assembly.21. The method of claim 13, wherein the heat source component is one ofa DCM and a MCM.